FIGS. 51(a) and 51(b) schematically illustrate a glass panel for forming a vacuum container or a multilayer glass panel (so-called double glass). Glass panel W includes a pair of glass substrates w3 and w4 having respective main surfaces 51 and S2 disposed oppositely with gap holding members Q (e.g., glass balls or resin balls) interposed between them so as to produce a gap of dimension g in the direction of the thickness of the panel and a bonding section n bonded to both of the main surfaces S1 and S2 at outer peripheral section (to be referred to outer peripheral gap hereinafter) k of the gap between the glass substrates w3 and w4 to seal the outer peripheral gap k and form an airtight chamber. The bonding section n is conventionally formed by glass frit. However, it has been proposed in recent years to form the bonding section n with a low melting point metal such as indium or solder from the viewpoint of improving the airtightness and the sealing quality including a low outgassing level. While the present invention is described below in terms of a technique of manufacturing a glass panel hereinafter, the following description by no means limits the scope of the present invention.
A glass panel for forming a vacuum container or a multilayer glass panel is manufactured by way of the following steps. (1) Preparing two rectangular glass substrates w3 and w4. (2) Supplying molten metal obtained by melting a low melting point metallic material such as indium or solder to the surface to be bonded of either one of or the both glass substrates along the outer peripheral edge or edges thereof in the form of a frame to form a bonding section. (3) Aligning the glass substrates w3 and w4 so as to make the bonding surfaces thereof exactly face each other. (4) Putting the two glass substrates w3 and w4 together and bonding them by way of the bonding section.
PTL 1 and NPL 1 disclose techniques relating to the above step (2) of supplying molten metal. More specifically, PTL 1 discloses an apparatus for applying a molten metal sealing/bonding material (which corresponds to a low melting point metallic material in this specification) to the surfaces to be bonded of a front substrate and a rear substrate (both are glass substrates) of a vacuum container of an image display apparatus, while applying ultrasonic waves thereto, in order to form a bonding member of the metal sealing/bonding material on the surfaces to be bonded for directly or indirectly bonding the front substrate and the rear substrate. NPL 1 discloses a technique of directly bonding bodies to be bonded having oxidized surfaces such as glass substrates, using a Pb—Sn based solder material containing one or more easily oxidizable elements selected from Zn, Al, Si, Ti and so on as bonding member, considering that such easily oxidizable elements enhance the bonding performance between a Pb—Sn based solder material and a glass substrate, and an ultrasonic soldering technique of removing the air bubbles existing at the contact interface of molten solder and a glass substrate by applying ultrasonic vibrations to improve the bonding performance between solder and a glass substrate.
Application of ultrasonic waves at the time when applying a molten low melting point metallic material (“a molten low melting point metallic material” will be referred to simply as “a molten metal” hereinafter in this specification unless noticed otherwise) provides an advantage of removing the air bubbles and the foreign objects existing at the surface of a glass substrate to improve the bonding performance at the bonding interface of the glass substrate and a bonding member.
Meanwhile, oxidation can proceed quickly to produce an oxide on the surface of a metal material regardless if it is in a solid phase or in a liquid phase. This phenomenon also occurs to a low melting point metallic material stored in the atmosphere in a solid phase. When the material is molten in order to form a bonding section, the oxide produced on the surface gets into the bonding interface of the bonding section and a glass substrate and/or is mixed into the molten metal material. The mixed oxide gives rise to defects at the interface of the glass substrate and the bonding section and also in the inside of the bonding section, which entail a problem of degrading the airtightness of the bonding section, the strength of the interface of the glass substrate and the bonding section and also the strength of the bonding section itself. Particularly, the above problem becomes remarkable when a bonding member is formed by utilizing ultrasonic vibrations as in the case of the techniques according to PTL 1 and NPL 1, because the oxide that is mixed into the molten metal is stirred by ultrasonic vibrations.
Exemplar techniques for dissolving the above problem are described in PTLs 2 and 3. PTL 2 discloses a configuration of disposing a pair of glass plates face to face and one above the other with a gap interposed between them and bonding the glass plates together by supplying a molten metal material that is stored in storage section with its surface held in contact with an inert gas atmosphere from the storage section to the outer peripheral gap to fill the gap for the purpose of providing a method of bonding a metal material and a glass plate, while suppressing generation of any oxide of the metal material and a method of manufacturing a glass panel that can be airtightly sealed on a stable basis by using the method.
PTL 3 discloses a method of manufacturing an image display apparatus that can suppress generation of oxide film on the surface of a metal sealing/bonding material and improve the wettability of metal sealing/bonding material relative to the sealing/bonding surface to make it possible to realize a complete sealing/bonding effect and a sealing/bonding material filling apparatus including a support table for positioning and supporting an object to be sealed/bonded having a sealing/bonding surface, a closed storage section for storing a molten metal sealing/bonding material, a nozzle for filling the sealing/bonding surface with the molten metal sealing/bonding material fed from the storage section and a filling head having a gas supply means for supplying stabilizing gas to the front end surface and its periphery of the nozzle to establish a stable gas atmosphere there.
PTL 4 discloses a different technique relating to the above-described step (2). PTL 4 describes a method of filling molten solder, using a metal supply cylinder having a discharge port for discharging a stored molten metal material (molten solder) and a lead-in plate for injecting molten solder into an outer peripheral gap disposed at a center section of the discharge port from the discharge port, the method including confining the gap formed between the discharge port and the corresponding end facets of a pair of glass plates to not greater than ten times the gap between the glass plates in order to reliably fill the outer peripheral gap with molten solder, suppressing any leak of molten solder if the gap between a pair of glass plates is small. PTL 4 describes that it is possible with the filling method according to the PTL 4 to fill molten solder into the outer peripheral gap, while suppressing the spread of molten solder to an unintended area if the gap of the glass plates is small.
Furthermore, PTL 5 discloses a still different technique relating to the above-described step (2). PTL 5 describes a method of manufacturing a glass panel including arranging spacers between a pair of glass plates to form a gap between the glass plates, filling a single molten metal material into a peripheral section of the gap to directly bond the pair of glass plates and the metal material and airtightly sealing the gap, wherein at least a part of a plate-shaped or rod-shaped guide for guiding the molten material is inserted into an outer peripheral gap in order to supply the molten metal material to the outer peripheral gap of the pair of glass plates.
PTL 5 also describes that, since a metal material injecting operation that is difficult particularly when the outer peripheral gap is narrow is accelerated and made easy by the guide and additionally the injecting speed is raised, the metal material and the glass substrates can be bonded directly with ease. PTL 5 further describes that a molten metal material can reliably be filled into an outer peripheral gap by appropriately selecting the size and the shape of the guide according to the outer peripheral gap.
Furthermore, PTL 5 describes a method of manufacturing a glass panel from a pair of glass plates having different sizes, the edges of one of the glass plates projecting beyond the corresponding respective edges of the other glass plates, wherein molten solder is made to permeate from the projection sections of the one of the glass plates toward the outer peripheral gap by means of a capillary phenomenon to fill the molten solder into the outer peripheral gap. PTL 5 also describes that such a capillary phenomenon can be produced by applying vibrations to at least either the molten solder or the glass plates to improve the wettability of the molten solder relative to the glass plates.
Additionally, a specific solder supply apparatus is described in Example 14 of PTL 5. The solder supply apparatus feeds molten solder by means of the self weight of the latter from a solder melting tank into an outer peripheral gap of a pair of glass plates disposed to form a gap of 0.2 mm between the main surfaces thereof to be bonded to a metal material by way of a pipe having an inner diameter of 3 mm, inserting a 0.15 mm-thick metal-made and plate-shaped guide fitted to the front end of the pipe into the outer peripheral gap by about 5 mm and filling molten solder into the outer peripheral gap along the outer peripheral edge of the glass plate. The width of the outer peripheral gap sealed by molten solder by means of the solder supply apparatus is about 5 mm from the outer peripheral edge and no problem is observed as a result of a leak test, a measurement of the heat transmission coefficient, a migration test of lead and a measurement of the oxygen content.